1. Field of the Invention
The present invention relates to a method for transmitting digitally coded analog signals which are combined in a coding block in a digital packet multiplex system in the form of equally long packets each having a beginning portion of specified length and a following useful portion for transmitting information-containing bits.
2. Description of the Prior Art
Digital transmission methods employing packets of equal length wherein the packet has a beginning identifying portion followed by a portion containing useful information to be transmitted will be employed in the digital transmission of high quality audio signals via broadcast satellites. As a result of the great flexibility afforded by such methods, packet multiplex methods enable accommodation of the greatest variety of digital signals in uniformally constructed data packets or blocks.
A method is known wherein a near-instantaneous compander is used during the digital coding of analog signals so that a number of uniformally stepped quantization scales or whose quantization differs by the factor of two can be utilized instead of the irregularly stepped quantization scale in the case of an instantaneous compander. In coding devices for high quality audio signals employing near instantaneous companding, a specific number of samples such as, for example 32, is first uniformally quantized and coded with the smallest of the provided resolutions. The resulting code words are digitally stored. The "fine" quantization is then either retained or converted to a "coarse" quantization as a function of the most significant of the stored code words and as a function of the number of bits per word which has been selected for transmission.
For example, if uniform quantization and 14 bit coding are executed in a first step, 4 bits of the 14 bits are, for example, after identification of the most significant code word, suppressed depending upon the significance of the most significant code word. A necessary condition for undertaking this method is that a scaling factor word consisting of a of bits to be additionally transmitted is attached to each of the blocks consisting of 32 code words, the scaling factor word providing information regarding the selected quantization scale. Clock synchronization between the transmit and receive sides of the system must also be present.
A system proposed the so-called NICAM 3 of the British Broadcasting Corporation (The Radio And Electronic Engineer, Volume 50, 1980, No. 10 at pages 519-530) employs a quantization characteristic with s=1024 intervals, n=10 bits per sample, and 32 samples per block. The European Broadcasting Union has proposed this method to the CCIR for application in broadcast satellite service.
Given the above described coding method, it is necessary that a block identifier be communicated to the receive side decoder, the block identifier indicating the beginning of the coding blocks.
It is also known to co-transmit one bit for reducing the influence of bit errors arising on the transmission link in addition to each coded sample. A parity check of, for example, the five most significant bits is executed with this bit. When a parity error as a consequence of a bit error in the protective bits of a code word is identified at the receive side, the faulty code word is suppressed and is replaced by an estimated value which can be acquired, for example, by linear interpolation between the code words which are chronologically immediately before and after the faulty code word. Mean bit error rates of up to approximately 10.sup.-5 may be tolerated by means of this simple protection against bit errors which, related to the above described coding example, increases the number of bits per sample to 11. Given more unfavorable bit error rates of, for example, 10.sup.-3 which may occur at the fringe or outside of a particular service region in the case of satellite broadcasting using digital audio transmission, it is necessary to provide substantially more additional transmission capacity for bit error protection, and to employ error-protecting codes, such as BCH code, HAMMING code, etc.
It is further known that continuous multiplexing with fixed time division of the pulse frame or time division of the pulse frame which is variable within strict limits or packet multiplexing may be employed for digital multiplex formation of a plurality of channels.
In the last instance, the useful information is accommodated in packets having a fundamental format as shown in FIG. 1. Each packet has a length w which is subdivided into segments x, y and z. A synchronization combination which is identical for all packets is contained in the segment x. The segment y contains address information which is different depending upon the channel or the type of service. The portion of the packet occupied by the segments x+y is also referred to as the head of the packet. The useful data is contained in the portion z of the packet. Because re-synchronization (by means of the x segment) is undertaken for every packet of a packet multiplex, both the length of the packets and the point in time of their arrival can be arbitrary. It has been shown that the transmission bit rate can only be occupied up to 90 to 95 percent by the useful data to the necessity of transmitting the packet head.
The transmission bit rate, however, can be more efficiently used when packet multiplexing is modified in a manner as described in the CCIR Green Book, Volumes X and XI Part II, Geneva, 1982, page 265. Only one uniform packet length is employed in this method. The packets appear in periodic succession, i.e., one packet immediately follows the preceding packet. The capacity required for synchronization purposes can be considerably reduced by this method because a special error protection for the periodically occurring synchronization combination need not be utilized. The synchronization means which is standard for continuous multiplexing can be utilized.
This modified packet multiplex, however, has the disadvantage that the coding blocks with 32 coded samples contain a different number of bits in the transmission of digitally coded audio signals having different outlay for the bit error protection, each sample containing, for example, 11 or 15 bits. Because it was previously assumed that the boundaries of the coding blocks should be simultaneously transmitted by means of the packet information, it was necessary to provide a complete series of coding blocks per packet. This unavoidably results in differing packet lengths given different error protection methods. Different demultiplexers are therefore necessary. A mixed transmission of signals having different error bit protection means, moreover, is not possible in the method described for modified packet multiplex in the CCIR Green Book.